Abstract Hematopoiesis is often viewed as a uniform process in which lymphoid and myeloid cells are continually generated in constant proportions throughout life. It is now evident that this is not the case and that, with increasing age, there is a decline in B cell development while myelopoiesis remains relatively intact. This in turn is thought to contribute to the increased susceptibility of the elderly to infection and the reduced efficacy of vaccination. The central hypothesis of this proposal is that the age-related accumulation of adipocytes and plasma cells in the medullary cavity impairs the capacity of the hematopoietic microenvironment to support lymphopoiesis. In contrast, myelopoiesis is either resistant to or is stimulated by these changing environmental dynamics. At birth most of the bone marrow is referred to as ?red? marrow to indicate that it is a site of active hematopoiesis. However, with increasing age there is an accumulation of adipocytes and the formation of non- hematopoietic ?yellow? marrow. The accumulation of bone marrow adipocytes is a gradual process, and we propose that as this occurs, lymphopoiesis is preferentially inhibited while myelopoiesis remains relatively intact. Experiments in Aim 1 will test this hypothesis using both in vivo and in vitro systems, and we will also analyze patterns of hematopoiesis in PPAR-?+/? mice, which have a highly significant diminution in bone marrow adipocytes, over time. The expectation is that age-related declines in B lymphopoiesis will be less severe in this strain due to their adipocyte deficiency. Bone marrow adipocytes provide a supportive niche for plasma cells. In view of their increase in number with age, we considered the possibility that plasma cells might also increase over time and found this to be the case. This has significant implications, as plasma cells are a source of numerous inflammatory cytokines known to inhibit B lymphopoiesis. Experiments in Aim 2 will test the hypothesis that this increase in plasma cells also contributes to age-related reductions in lymphocyte development. We will build upon preliminary data showing that plasma cells co-localize with hematopoietic stem cells (HSCs) in the bone marrow and evaluate the potential of plasma cell derived signals to skew the developmental potential of HSCs towards myelopoiesis. We will also generate Cd19-Cre;flPrdm1fl mice which lack plasma cells and determine if declines in B lymphopoiesis in these animals are less precipitous over time. A key question from a translational perspective is whether lymphopoiesis can be rejuvenated in old individuals. The experiments in Aim 3 will test the ability of various interventions, which include drugs that inhibit adipocyte formation and antibodies that deplete plasma cells, to rejuvenate B lymphopoiesis in the elderly. Additional experiments will determine if these interventions target HSCs and affect patterns of gene expression in them. Taken together, the data obtained in the proposed studies will increase the understanding of how aging affects hematopoiesis and identify interventional targets that may have translational potential for rejuvenating immunity in the elderly.